The present invention relates to an image reading apparatus. More particularly, it relates to an image reading apparatus comprising color line image sensors having a color filter for every pixel for use in color scanners and color copying machines.
Recent color scanners and digital color copying machines generally use line image sensors such as CCD's for reading images.
The line image sensors have a color filter arranged for every pixel to read color information. There have been many proposals to arrange the color filters.
FIG. 12 shows an example of a prior art line sensor arrangements. In FIG. 12, where R denotes a red pixel, G is a green pixel, and B is a blue pixel, a main scanning line has filters of the same color arranged therein, and two other scanning lines have different color filters arranged therein in a subscanning direction.
The arrangement described above has the advantage that the color filters can be fabricated and mounted rather easily and laterally long filters of the same colors may be easily prepared.
However, in the method shown in FIG. 12, as other color information with respect to a pixel exists in the subscanning direction, preceding and succeeding information has to be read as a scan is made. To do this, the number of the line memories has to be increased.
In order to solve such a problem, the inventors proposed a CCD line sensor in which, as shown in FIG. 11, red and green pixels were alternately arranged on a line, while blue pixels were continuously arranged on a different line. Note that a white filter instead of a blue filter may be used. In this case, the blue information can be obtained from the subtraction of the other two signals.
The arrangement of FIG. 11 is desirable because number of line memories can be reduced. However, the inventors found that it had the disadvantage that the resolution in the main scanning direction might be degraded as compared to that of the three-line method discussed above. As the red information, for example, is given for every other pixel, the resolution in the main scanning direction is halved.
The inventors estimated the lost information with use of a linear interpolation method illustrated in FIG. 3. In FIG. 3, where the order of a pixel of interest in the main scanning direction is denoted by i, green information G(i) on a green (G) channel can be obtained quickly. However, red information R(i) cannot be obtained. To overcome this, adjoining red informations R(i-1) and R(i+1) are brought in to make a mean signal R(i) of both signals. Of course, other informations R(i.+-.2d+1, where d is an integer) can be used to increase the resolution. In any case, the other red informations are estimated from the known red information only.
After the inventors considered the linear interpolation method, they found that there is a limitation in the interpolation effect, particularly in reproducing a monochrome image in which false colors are mixed in the reproduced image, resulting in an increase of errors in a reproduced image compared with an original image. At a boundary where the region of a character changes to a non character region, for example, the original image changes from black to white as in a digital mode. However, the linear interpolation estimation does not change the information values of red and green to zero, but causes recognition of false colors.